1. Field of the Invention
The present invention relates to a motor protection circuit breaker, a so-called ‘manual motor starter’, and particularly, to a switching mechanism for a motor protection circuit breaker capable of reducing a force required to operate a handle, and capable of enhancing the reliability to break a circuit.
2. Background of the Invention
A motor protection circuit breaker, so-called a ‘manual motor starter’ (MMS) is an apparatus having a switching function for starting or stopping an electric motor, and having a protection function for protecting the electric motor by automatically breaking a circuit upon the occurrence of an abnormal current such as a short current on the circuit.
The conventional switching mechanism for a motor protection circuit breaker comprises a handle, a link mechanism, a push link, and a movable contact mechanism.
The handle provides a means to manually operate the motor protection circuit breaker to an ‘ON’ position (conducting position), or an ‘OFF’ position (circuit breaking position).
The link mechanism transmits an operating force by the handle for the ‘ON’ position or ‘OFF’ position.
The push link is rotated by the link mechanism, thereby providing the operating force to open and close the circuit.
The movable contact mechanism is movable to a position contacting a fixed contact, or a position separated from the fixed contact according to whether a pressure applied from the push link exists or not. Here, the movable contact mechanism includes movable contact plates and a cross bar. The movable contact plates are provided in three in number, for example, for three-phases (R phase, S phase, and T phase) alternating current. Contactor is attached to each of the movable contact plates for three-phase. The cross bar is a means to transmit an operating force to open and close the circuit generated by the push link, to the plurality of movable contact plates.
The conventional switching mechanism for a motor protection circuit breaker further comprises a latch and a trip spring for automatically breaking the circuit upon the occurrence of an abnormal current.
The latch is a means connected to the link mechanism, and displaceable to a position for locking the link mechanism or a position for releasing the locked state of the link mechanism.
The trip spring is a means to provide a driving force for breaking a circuit. Contact pressure springs for maintaining a contact pressure with the fixed contacts are installed below the movable contact plates for three-phase, respectively.
In the case of rotating the handle to an ‘OFF’ position by a user, the rotational force of the handle is transmitted to the push link via the link mechanism. As the push link is rotated, the cross bar is downwardly pressed. Accordingly, the cross bar overcomes the contact pressure by the contact pressure springs, and downwardly presses the movable contact plates for three-phase. This may allow the movable contacts attached to the movable contact plates for three-phase to be separated from the fixed contacts. As a result, an ‘OFF’ state that the circuit is artificially broken is implemented.
Upon the occurrence of an abnormal current such as a short current on the circuit, the latch is released by a trip mechanism including a trip coil (not shown) for generating a trip driving force by being magnetized by the abnormal current. As a result, by elastic energy discharged from the trip spring, the push link connected to the trip spring is rotated. The push link being rotated downwardly presses the cross bar. Accordingly, the cross bar overcomes the contact pressure by the contact pressure springs, and downwardly presses the movable contact plates for three-phase. This may allow the movable contacts attached to the movable contact plates for three-phase to be separated from the fixed contacts. As a result, a ‘TRIP’ state is implemented.
In the case of rotating the handle to an ‘ON’ position by the user, the rotational force of the handle is transmitted to the push link via the link mechanism. As the push link is rotated in an opposite direction to the ‘OFF’ direction, the cross bar is released. Accordingly, the movable contact plates for three-phase are upwardly moved by the contact pressure by the contact pressure springs. This may allow the movable contacts attached to the movable contact plates for three-phase to contact the fixed contacts. As a result, an ‘ON’ state that the circuit is closed is implemented.
In the conventional switching mechanism for a motor protection circuit breaker, in order to implement a closed circuit (conducting state) that the movable contacts contact the fixed contacts as the cross bar and the movable contact plates for three-phase upwardly move, the cross bar and the push link have to be separated from each other with a predetermined distance therebetween in an ‘ON’ state. Accordingly, in order to rotate the push link so as to be separated from the cross bar by a predetermined distance, a large operating force of the handle was required, the operating force large enough to overcome an elastic force of the trip spring, and enough to rotate the push link. Furthermore, in order to manually operate the conventional switching mechanism for a motor protection circuit breaker to an ‘OFF’ state from an ‘ON’ state, also required was an operating force large enough to overcome an elastic force of the trip spring, and enough to rotate the push link toward an ‘OFF’ direction. That is, there has been required a large force for a user to operate the handle in an ‘ON’ or ‘OFF’ direction.
The conventional switching mechanism for a motor protection circuit breaker has the following problems.
Firstly, it was difficult to decrease an operating force of the handle while satisfying a condition that the cross bar and the push link have to be separated from each other with a predetermined distance therebetween, in an ‘ON’ state.
Secondly, due to an incomplete releasing operation of the latch, the switching mechanism may perform an incomplete circuit opening operation (trip operation). This results in a problem that breaking the circuit upon the occurrence of an abnormal current may fail. Accordingly, has been required a method for reducing an operating force of the handle, and for automatically or manually breaking the circuit with an enhanced reliability.